1. Field of the Invention
This invention relates to a method for converting inorganic polysilazanes, into useful preceramic and ceramic materials having very little or no available carbon for use in conjunction with the fabrication of electronic circuit boards, electronic device coatings, broad band radomes and the like. Additionally, the process proceeds at low temperature conditions, thereby allowing long term high temperature sensitive materials, such as ultra pure fused quartz fiber, to be used in composite fabrication.
2. Brief Description of the Prior Art
Electronic circuit boards, electric device coatings, broad band radomes and the like require low dielectric and low dissipation properties in order to minimize the loss of microwave energy due to heating. Broadband radomes, in addition, must be capable of working throughout large regions of the microwave spectrum. Typically, prior thermal requirements for such devices have been satisfied using a composite of high performance organic polymers in conjunction with fused quartz fabric for radomes. The properties required from the fabric are that it have good structural integrity and be transparent for microwave transmission, ultra pure fused quartz being the preferred such material since it has the best combination of the desirable properties.
The requirements for the next generation of radomes are very stringent for short term, very high temperature exposures, such requirements including the ability to sustain temperatures as high as 1600.degree. F. (870.degree. C.) for 5 minutes and 2300.degree. F. (1260.degree. C.) for a few seconds without significant deterioration. These conditions substantially exceed the capability of the prior art of organic polymers. The organic polymers are readily consumed by pyrolysis and oxidation destruction at these temperatures. As a result, inorganic materials must be used as the materials of choice. However, fused quartz fabric, which must be used in the composite to insure meeting the electrical requirements, cannot be subjected to the high temperatures required in the processing of typical inorganic ceramic systems, these temperatures generally being in excess of 1000.degree. C. for a period of hours. Exposure of quartz to temperatures of 1000.degree. C. for one hour have resulted in a reduction to 25% of the original tensile strength. Damage to the quartz has been encountered at temperatures as low as 750.degree. C. over a longer period of time.
A solution by this invention to the above described problem for these new radome requirements relates to the use of high temperature-enduring inorganic polymer systems for radome fabrication. By using organo-inorganic polymers (precursors), such as, for example, perhydropolysilazane and low carbon containing polysilazanes such as methylhydridocyclosilazane, or a mixture of these, processing can be conducted at lower temperatures such as 700.degree. C. and below to achieve a near ceramic state material. The use of these near ceramic state materials in higa velocity radomes, for example, would result in further conversion of the material to a ceramic material during the high temperature trajectory, but this high temperature condition would be experienced over a very short period of time, thereby resulting in no quartz damage. While inorganic polymers can resolve the high processing temperature causing damage to quartz, the fabric material of choice, they introduce new difficulties which are resolved in accordance with the present invention.
One of these difficulties is that most inorganic polymers have organic components within their system. Upon processing to temperatures of 400.degree. C. and above, the organic components in many cases decompose to some free elemental carbon. This carbon is very damaging to the electrical properties of the final ceramic material.
Additionally, the inorganic polymer conversion from polymer to ceramic is generally defined as a low yield conversion. Yield is defined as the ratio of the weight of final material to the original polymer. Due to the low yield, a significant volumetric conversion occurs, resulting in matrix cracks, voids and pull away from any fibrous material which may be in the composite. The net effect is a reduction in structural properties. This reduction in structural properties requires numerous reinfiltrations to add replacement material to the depleted areas. This then requires a repeat of the high temperature process steps to densify the composite. This repeated processing is very damaging to the quartz used to accomplish the ideal electrical properties. This problem is addressed by the present invention by increasing the molecular weight and crosslinking of the polymer with additional changes such as the addition of a high pressure compaction processing step and the addition of resin fillers.
It is apparent that the below described new processing steps are required in order to meet the specifications for the new class of radomes and the like.